The present invention is a method and apparatus suitable for implementing improved compact dot growth (CDG) in a printing system. Using the improved compact dot growth method, image data discarded from an input image during an encoding process may be subsequently reconstructed in a manner which serves to replenish a substantial amount of the discarded image data and minimize degradation of an output representation of the image. The general purpose of a compact dot growth operation is to eliminate or reduce as many "gray" exposures as possible in the output image. It is known that "gray" exposures may be produced in a pulse-width position-modulated output device by varying the pulse width and position of the exposure pulse (see e.g., Cianciosi et al., U.S. application Ser. No. 08/118,858, filed Sep. 10, 1993). Ideally, the "gray" exposures are eliminated by shifting the partial pixel pulses to a neighboring black pixel, thereby "growing" or elongating the exposure period of the neighboring pulse. Similarly, adjacent pairs of gray pulses can be merged, producing exposure profiles that result in xerographically better improved stability and/or uniformity on the output print. This process eliminates the "gray" exposures which are shorter and isolated, and which tend to be less stable xerographically than black pulses. While the general process of employing a compact dot growth operation has been described in pending application Ser. Nos. 08/496,556 by Parker and 08/496,654 by Coonan et al., the present invention improves on the described operation by recognizing opportunities to improve xerographic stability when a plurality of "gray" pixel exposures are present adjacent to one another but between either black or white pixels. Moreover, the present method employs only a single dimension template with which it accomplishes recognition of those pixels requiring correction.
It is known that the output of an image can be enhanced significantly through the use of a high addressability output device. For example, by using a printing machine with a capability of storing or buffering scanned data prepared at a resolution of 600.times.600.times.3 spots/inch (spi) a corresponding output of 1800.times.600.times.1 spi can be obtained. While such high resolution is desirable, storing image data at 600 .times.600)&lt;3 spi is burdensome in a printing machine with the limited amount of memory found in many digital printing systems. While various printing systems advantageously exploit disk storage and associated compression schemes (e.g., Xerox DocuTech, and U.S. Pat. No. 5,047,955, to Shope et al. (Issued Sep. 10, 1991) it is, for purposes of outputting a job, necessary to store or buffer the job in system memory; particularly when the job must be precollated. Hence it is desirable to store or buffer a job in system memory of a high addressability printing machine at a first resolution and output it at a second resolution, where the first resolution is significantly less than the second resolution.
For example, as disclosed by Coonan et al. storing or buffering a job at less than 600.times.600.times.3 spi is accomplished by discarding image data. Naturally the discarding of image data may lead to a degradation of image quality when reproducing the image data. It is, therefore, also desirable to provide an apparatus that facilitates a technique or process in which at least a part of the discarded image data could be reconstructed, where image degradation is minimized using a compact dot growth process.
Using morphological, template-matching techniques the present invention facilitates the identification of specific pixel structures within a bitmap image. Once identified, the structures may be "tuned" so as to appear in the fashion desired when rendered by a raster output scanner (ROS) or similar image output terminal (IOT).
Heretofore, a number of patents and publications have disclosed template-based filters for image enhancement and resolution conversion, the relevant portions of which may be briefly summarized as follows:
U.S. Pat. No. 4,544,264 and U.S. Pat. No. 4,625,222 to Bassetti et al., disclose enhancement circuits suitable for use in a laser based electrophotographic printing machine. The enhancements are directed at modifying the digital drive signals used to produce the image, including smoothing digitized edges and broadening fine lines in both the horizontal and vertical directions. Leading and trailing edge signals, in both directions are provided to potentially print each black pixel or line as a series of three pixels, a gray leading pixel, overlapped by a central black pixel, which is in turn overlapped by a gray trailing pixel. A similar process is applied for scan lines as well. The series of signals are recombined to effectively control the voltage and current levels of a laser driver.
U.S. Pat. No. 4,847,641 and U.S. Pat. No. 5,005,139 to Tung, teach print enhancement circuitry for a laser beam printer. The bit map of a region of the image to be output is compared to a number of patterns or templates. When a match is detected, a section of the bitmap that was matched is replaced with a unique bitmap section designed to compensate for errors. The replacement bitmap section may include predetermined shifting of some dot positions to compensate for the error in the original bitmap section.
U.S. Pat. No. 5,029,108 to Lung teaches an edge enhancement method and apparatus for dot matrix devices wherein a group of gradient mask matrices are applied to a matrix of pixels surrounding a "to be adjusted pixel" so as to determine the existence of an edge and the direction of the brightness change. Once determined, the factors are used to generate a code used to modify the to be adjusted pixel in order to enhance the smoothness of a segment transition.
U.S. Pat. No. 5,408,329 to Mailloux et al., discloses the use of digital darkness control or pixel stretch techniques in electronic imaging devices regardless of differences in development systems (e.g., write-white or write-black). The inventors note that it is important to match images or have images look alike regardless of the particular system in a given machine that is used in developing the image. Accordingly, the invention provides an electronic adjustment to an original image in order to compensate for various development systems as well as to compensate for quality settings within a given development system. Employed are sets of decoding rules applicable to 2.times.2 or 3.times.3 arrays of image pixels to accomplish darkness adjustment for a given pixel matrix.
U.S. patent application Ser. No. 07/513,415, and the corresponding Japanese laid-open patent publication 4-227584 published Aug. 17, 1992, to Maiiloux et al. disclose a method to enable the conversion of binary image data originally generated at a lower resolution into representative binary image data of a higher resolution, wherein the conversion ratio, or magnification factor, is an integer value. Included within the resolution magnification invention are methods for smoothing the interpolated output image and thereby reducing objectionable visual characteristics observable in digitally encoded data using conventional magnification techniques.
A number of patents and publications are summarized in Torrey Pines Research, Behind Hewlett-Packard's Patent on Resolution Enhancement.TM. Technology, (Becky Colgan ed., BIS CAP International, 1990) pp. 1-60, including concepts associated with resolution enhancement.
James C. Stoffel et al. in A Survey of Electronic Techniques for Pictorial Image Reproduction, IEEE Transactions on Communications, Vol. COM-29, No. 12, December 1981 discloses image processing algorithms that can be used to transform continuous tone and halftone pictorial image input into spatially encoded representations compatible with binary output processes. A set of image quality and processing complexity metrics are also defined so as to evaluate a number of image processing algorithms with respect to their ability to reproduce continuous tone or halftone pictorial input.
Mathematical Morphology in Image Processing, pp. 43-90 (Edward R. Dougherty ed., Marcel Dekker 1992) describes efficient design strategies for the optimal binary digital morphological filter. A suboptimal design methodology is investigated for binary filters in order to facilitate a computationally manageable design process.
M. Anzai et al. in "A New High-Resolution Halftone Printing Method For an Electrophotographic Laser Printer," Society for Imaging Science and Technology--41st Annual Conference, Arlington, Va., May 1988, pp. 343-346, teach the difficulty in accomplishing halftone rendering with pulse width modulated laser light. Proposed is a neighboring dot connection method wherein dots of neighboring picture elements are joined to keep the dot information in the picture elements.
In accordance with the present invention, there is provided a compact dot growth method for selectively controlling the exposure of a xerographic printing system capable of producing printed output from a print engine in response to a digital image, consisting of a plurality of digital image signals each digital image signal indicating one of a plurality gray exposure levels as a multi-bit value, comprising the steps of: storing, in memory, a one-dimensional segment of the digital image signals forming the digital image; identifying, using a template matching operation applied to the stored digital image signals, a multi-bit value associated with a pixel position to be selectively controlled; for the pixel position identified to be selectively controlled, producing a pixel code representative of an exposure duration and position shift to be applied to the pixel position to be selectively controlled, and directing the print engine to shift the exposure position in response to the pixel code.
In accordance with another aspect of the present invention, there is provided an image processing apparatus for processing a bitmap image consisting of a plurality of input image signals representing a bitmap image, comprising: a one-dimensional context memory for storing a plurality of multi-bit image signals from a single raster of the bitmap; a template matching circuit for comparing the multi-bit image signals stored in said one-dimensional context memory with a plurality of templates, said template matching circuit outputting a signal indicating a match between the stored image signals and a template; and a pixel code generator, responsive to said template matching circuit, for generating a pixel code in response to the signal indicating a match between the stored multi-bit image signals and the template, said pixel code indicating the exposure duration and shifted location within a pixel period for a target pixel within the context memory.
In accordance with yet another aspect of the present invention, there is provided a printing system for processing a bitmap image consisting of a plurality of gray input image signals at a first resolution to be represented as a binary bitmap image at a resolution greater than the first resolution, comprising: a one-dimensional memory buffer for storing a portion of a single raster of the gray image signals at the first resolution; pattern matching logic circuitry for identifying, within the stored gray image signals at the first resolution, a target pixel requiring modification prior to exposure of a binary pixel at a second resolution greater than the first resolution, said pattern matching logic circuitry producing a signal whenever the target pixel requiring modification is identified; and a programmable device, responsive to the signal generated by the pattern matching logic circuitry and the target pixel image signal, for producing an output signal for driving the printing system, the output signal resulting in the production, by the printing system, of a binary exposure pulse shifted within a pixel period defined by the first resolution.
One aspect of the invention is based on the observation of problems with conventional printing systems. Such systems, although they may be capable of rendering images received in a page description language (PDL) such as Interpress (Xerox Corp.) or Postscript (Adobe Corp.), typically render the image in accordance with the appearance determined as a result of the particular marking engine employed. However, such systems may result in user dissatisfaction because of an expectation of the appearance, based in part upon the WYSIWYG document preparation software and/or prior prints produced on other printers (e.g., dedicated workstation or desktop printers).
This aspect is based on the discovery of a technique that improves the stability of xerographically generated output. This technique can be implemented, for example, by circuitry that is also employed to accomplish compact dot growth after resolution reconstruction of an encoded image. The present invention finds particular use for complementing other compact dot growth techniques so as to assure xerographic stability and improve the uniformity of an output print. The compact dot growth circuitry of the present invention is also simplified, relying solely on a one-dimensional "window" of pixels surrounding a pixel of interest. Thus, only a partial raster of image data need be buffered to implement the techniques of the present invention. A machine implementing the invention can include a laser-based electronic printing system, wherein the laser beam intensity or exposure level may be modified via pulse width position modulation (PWPM) in response to the signals representing the tuned or reconstructed image.